Canine hip dysplasia(HD) is a genetic disease which is inherited in a rather complex manner
because it is influenced by more than one gene.That
is its mode of inheritance is polygenetic.It was first recorded back in the 1930s but its incidence has
increased as the popularity of breeding and showing dogs has increased.Whether this is coincidence or whether there is
some relationship has not been established.In
a number of breeds the disease is present in some degree in over 80% of individuals.

The hip joint is a ball
and socket joint with the ball (femoral head) being on the proximal end of the thigh bone
(femur) and the socket (acetabulum) being on the pelvis.If the hip joint is normal the ball is a neat fit in the socket.Generally all new born puppies have normal hip
joints but in those that have HD an abnormality develops during growth.The problem appears to be in the growing of
the socket.For the socket to grow bigger as
the dog grows requires quite a complex process because it is difficult to grow a hole.With the acetabulum this is achieved by the three
bones that make up one side of the pelvis (ilium, ischium and pubis) joining at the
acetabulum thus forming a complex pattern of growth zones.If the acetabulum is to develop correctly the growth rate at these different
zones has to be precisely matched.If it is
not, the acetabulum will become distorted so that the ball is no longer an accurate fit
and this is hip dysplasia.Some recent
research suggests that dogs with HD have problems with other bone growth zones but because
these are mainly involving single bones they do not cause problems.

Hip dysplasia in
itself does not commonly cause lameness.It
only does so if it is of such a degree that the hip dislocates and this is uncommon.So many dogs, particularly younger dogs, with hip
dysplasia show no signs of lameness.This
combined with the fact that it is a developmental abnormality is the reason why we have to
resort to assessing dogs for hip dysplasia from an X-ray taken when the dog is at least 12
months of age.

With
dogs that have HD the ball is not a neat fit in the socket so the joint is subject to
excessive wear and tear.This excessive wear
and tear results in the development of degenerative joint disease (arthritis).It is the arthritis that causes the lameness.Other factors can impact of the degree of wear and
tear the joint is subjected to.The two
main ones are the weight of the dog and the amount and type of exercise the dog takes.Obviously the heavier the dog the more stress is
put on the hip joint so large heavy dogs are more likely to become lame because of hip
dysplasia than small light ones.Dogs
that get a lot of exercise at fast gaits such as galloping behind a car or bicycle and
dogs that do a lot of jumping or turning and stoping such as when fetching balls or sticks
stress the hip joints and are therefore more likely to become lame.A normal hip joint can sustain these sorts of
stresses without developing arthritis.So
the age at which a dog with HD shows signs of lameness and the severity of the lameness is
influenced by an inherited component. That is the degree of developmental abnormality of
the joint. Then there is the environmental influence which is the degree of stress the
joint is subjected to.

When it finally became
generally recognised in the 1960s that hip dysplasia was an inherited disease dogs were
screened for its presence by taking an X-ray of the pelvis. From the X-ray the degree of
HDwasgraded
Normal, Near normal , Slight, Moderate etc. but this was a highly subjective method of
assessment so there was variation in interpretation from one assessor to another.In the late 1960s Prof DD Lawson and

DrMB Willis evolved the scoring system used in the UK, Australia, New Zealand and Singapore.Thisgives a more objective assessment of the hip
joints.The standardisation of the scores
awarded is further contributed to by only accepting scores
awarded by specialist veterinary radiologists .